Eighty-nine strains of Lactobacillus delbrueckii subsp. lactis isolated from Italian hard and semi-hard cheeses and artisan starter cultures were characterised by phenotypic and genotypic methods. Phenotypic diversity was evaluated by studying biochemical characteristics (i.e. acidifying and peptidase activities) of technological interest. Genotypic diversity was evidenced by RAPD-PCR and pulsed field gel electrophoresis (PFGE). Phenotypic characterisation indicated a wide variability of the acidifying activity within Lact. delbrueckii subsp. lactis. Although the data was variable, it allowed us to evidence groups of strains with different acidifying properties, especially in terms of acidification intensity. Concerning peptidase activity, Lact. delbrueckii subsp. lactis showed a homogeneously high x-prolil-dipeptidil-aminopeptidase activity and a considerable but more heterogeneous lysil-aminopeptidase activity. The increased resolution obtained by the use of two molecular typing techniques, i.e. RAPD-PCR and PFGE, allowed to widen the level of strain heterogeneity. Technological and ecological pressures are determinant in selecting Lact. delbrueckii subsp. lactis sub-populations which are more functional to the different cheese technologies.